Edition Published by Infineon Technologies AG Munich, Germany 2011 Infineon Technologies AG All Rights Reserved.

Transcription

1 Application Note AN213 Revision: 0.6 Date: LED Driver & AF Discretes

2 Edition Published by Infineon Technologies AG Munich, Germany 2011 Infineon Technologies AG All Rights Reserved. LEGAL DISCLAIMER THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices, please contact the nearest Infineon Technologies Office ( Warnings Due to technical requirements, components may contain dangerous substances. For information on the types in question, please contact the nearest Infineon Technologies Office. Infineon Technologies components may be used in life-support devices or systems only with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered.

3 Application Note AN213 Revision History: Previous Revision: Previous_Revision_Number Page Subjects (major changes since last revision) Application Note AN213, / 20

4 ILD4001 List of Tables Table of Contents 1 Introduction... Fehler! Textmarke nicht definiert. 2 Application Information Characteristic Graphs for different Inductors, no. of LEDs, Rs Evaluation Board and layout Information Equations for estimating switching frequency or Inductance List of Figures Figure 1 ILD Figure 2 Schematic of the demonstration board... 8 Figure 3 Measurement setup for measuring Vsense voltage w.r.t. Vs pin... 9 Figure 4 Vsw, Vsense and VLED(-), Vs= Figure 5 Switching Freq. vs Input Voltage,Vs... 9 Figure 6 Dimming Waveforms Figure 7 Maximum Contrast Ratio vs Dimming frequency (100:1=1% duty) Figure 8 Analog Dimming Characteristic Figure 9 Additional circuitry for EN/PWM < 1.0V Figure 10 Alternative circuitry for EN/PWM < 1.0V Figure 11 Vsense vs Vs (Rs = 0.159, L = 68uH) Figure 12 Vsense vs Vs (Rs = 0.078, L = 33uH) Figure 13 ILED vs Vs (Rs = 0.159, L = 68uH) Figure 14 ILED vs Vs (Rs = 0.078, L = 33uH) Figure 15 Efficiency vs Vs (Rs = 0.159, L = 68uH) Figure 16 Efficiency vs Vs (Rs = 0.078, L = 33uH) Figure 17 ILED vs Ambient Temperature Figure 18 Efficiency vs Ambient Temperature Figure 19 Solder Point Temperature vs Ambient Temperature Figure 20 Photograph of Demo Board using BSP381S external MOSFET (size of PCB: 50mm x 30mm) Figure 21 PCB Layer Information Top View Figure 22 PCB Layer information Bottom View (unflip) Figure 23 Thermal Resistance of PCB-FR4 versus Ground Copper Area Figure 24 Thermal Resistance List of Tables Table 1 Demo Board for ILD Table 2 Bill-of-Materials... 8 Table 3 Percentage of max LED current vs DC voltage at PWM pin Table 4 Steps to calculate the switching frequency for the demo boards Application Note AN213, / 20

5 1 ILD4001 Step down LED Controller for high power LEDs 1.1 Features Wide Input Voltage Range: 4.5 V V Capable to drive N-channel Power FETs that provide >3A of output current 120 C Temperature shut down mechanism Switching frequency up to 500kHz Analog dimming possible Typical 3% output current accuracy Overvoltage protection Minimum external component required Small Package: SC-74 Figure 1 ILD Applications LED Controller for indoor and outdoor illumination LED Retrofit, e.g. MR16 Halogen replacement Retail, Office and Residential high power downlights Architectural lighting Stage lighting High power spot lights 1.3 Description This document contains informations about the LED-Less Demonstration Board for ILD4001. Please refer to the datasheet for the pins descriptions, functions descriptions and specifications. The ILD4001 is a hysteretic buck LED controller IC for industrial applications realized in a bipolar IC technology. The LED Controller is capable to drive external bipolar or MOSFET power transistors by using the internal pushpull output stage. ILD4001 maintains a constant current through a string of LEDS as long as the input voltage exceeds the sum of the forward voltages of the LEDs in the string by at least 3V. The maximum input voltage for this demonstration board must not exceed 30V due to the board is optimizing for the 30V operation. If there is a need to test the board with a maximum supply voltage of 40V, please replace the schottky diode SD1 and the external MOSFET T1 with a suitable breakdown voltage. The precise internal bandgap stabilizes the circuit and provides stable current conditions over temperature range. Furthermore, over voltage protection and temperature shut down mechanism enforce the IC to protect attached LEDs. The board includes an EN/PWM input terminal for digital or analog dimming control signal. The demonstration board is designed to operate at ambient temperatures up to 100 C. The complete demonstration board schematic is shown in Figure 2. Typical waveforms and performance curves are shown in Figures 4 to 8. Although a wide variety of LED combinations and currents can be driven with the ILD4001, the sense-resistors have to be altered to achieve the desire LED current and inductance has to be changed to attain recommended switching frequencies below 500 khz. Application Note AN213, / 20

6 Table 1 Board Name Demo Board for ILD4001 R1,R2,R3 /Ω L1 /µh External MOSFET Vs /V Suitable number of LEDs Typical Switch. Freq. /khz Measured Vrsense = Vs - VLED+ /V LED Average Current /A Ambient temp. less than 700mA BSR302N mA BSP318S The above measured values are for typical case only. / C Check List before powering up Before powering on the ILD4001 demonstration board, please verify the following: Be sure that each LED can conduct 1000mA dc current within its safe region of operation. Make sure that the input voltage supply is less than 30V. Select the appropriate mode for EN/PWM: to enable the ILD4001, please force the EN pin terminal to 3V or more, and to select analog dimming, supply a dc source (0 to 3V) to PWM pin terminal, or to select PWM dimming, supply a PWM signal source (0 to 5V) with frequency within range of (200Hz to 5 khz) to PWM pin terminal. For the case when operating the EN/PWM pin less than 0.7V, it is requiring to insert a 10K~100K ohm resistor between the gate of the external power MOSFET and ground External MOSFET The external MOSFET T1 is required to drive the LEDs in the ILD4001 application. There are a few factors to consider while choosing the suitable external MOSFET. First, choose the correct voltage and current rating of the MOSFET. Please ensure the VDS breakdown and current capability is sufficient, and ensure that the external MOSFET working within the SOA region of DC mode. Second, the logic high level from ILD4001 is 5V and the external MOSFET must be able to be driven with a 5V gate voltage. Third, choose a low Ron MOSFET. It can improve the efficiency of the system. The BSR302N and BSP318S are recommended Capacitor C20 for Ripple Reduction This component C20 is optional and not installed on the standard demo board. This capacitor can help to reduce LED ripple current. Recommended to use low ESR 2 capacitor and its rated voltage must be higher than the maximum input voltage Connection of LEDs The ILD4001 demo board includes a 3-pin SIP 3 connector for the anode connection (LED +) and a 2-pin SIP connector for the cathode connection (LED -) of the LEDs in series. The anode connection is labeled as Con1-3 and cathode connection is labeled as Con2-1 on the board. 1 Demo board without heatsink 2 Equivalent Series Resistance 3 Single In-line Package Application Note AN213, / 20

7 1.3.5 PWM Dimming The PWM terminal on the PCB is an input for the pulse width modulated (PWM) signal to control the dimming of the LED string. The PWM signal s logic high level should be at least 2.5V or higher. The period of this PWM signal should be higher than 200us. For the default demo board circuit, a dimming frequency less than 300Hz is recommended to maintain a maximum contrast ratio of at least 100:1. The maximum contrast ratio is shown on Figure 7, and the minimum is based on the measured average LED current at 3db above/below the linear reference. The maximum contrast ratio depends largely on the rise time of the inductor current, and hence is dependent on input voltage, inductor size, and LED string forward voltage. In addition, if C20 is installed, the maximum contrast ratio or DIM frequency will be further reduced. Please insert a 10KΩ resistor between the MOSFET s gate and ground. This is due to the output stage of ILD4001 become high impedance state when the PWM signal is lower than 0.7V. With the 10KΩ resistor, a dimming PWM frequency up to 5 KHz is possible Open Circuit of terminals LED+ and LED- If the LED array is disconnected or fails with open state, the ILD4001 will operate at 100% duty cycle. The output voltage (at LED+) will rise to the level of the input voltage. The other output terminal (LED -) will fall to ground. Note that under the above said condition; please avoid reconnecting the LED array between LED+ and LED- terminals without powering down first. This precaution is to avoid excessive surge current that may damage the LEDs. Application Note AN213, / 20

8 2 Application Information 2.1 Schematic Figure 2 Schematic of the demonstration board Table 2 Bill-of-Materials Symbol Value Unit Size Manufacturer Comment L1 *see Table 1 uh 10.4x10.4mm / EPCOS / Shielded Power Inductor 8x8mm TAIYO YUDEN R1 *see Table 1 Ω 1206 Part of the current sense resistor R2 *see Table 1 Ω 1206 Part of the current sense resistor R3 *see Table 1 Ω 1206 Part of the current sense resistor R10 0 Ω 0805 Jumper SD1 BAS3020B SOT363 INFINEON Medium Power AF Schottky Diode 2A 30V IC1 ILD4001 SC-74 INFINEON Hysteretic Buck controller and LED driver T1 *see Table 1 PG-SC-59 / PG-SOT-223 INFINEON OPTIMOS 2 / SIPMOS Small Signal Transistor C uf 1812 Ceramic, 50V Application Note AN213, / 20

9 2.2 Recommended method to measure Vsense w.r.t. Vs pin Figure 3 Measurement setup for measuring Vsense voltage w.r.t. Vs pin By probing Vsense pin voltage with reference to Vs pin, it facilitates the observation and measurement of the ripple and average of Vsense voltage at the same time with Oscilloscope set to DC coupling, and without offsetting the DC voltage. This is shown in Figure Measured Graphs of the demonstration boards Unless otherwise specified, the following condition labels apply: Condition: Vs=12V, Ta=25 C, 700mA demo board, LEDs-in-series=3 x LUMINUS SST-50W Figure 4 Vsw, Vsense and VLED(-), Vs=12 Figure 5 Switching Freq. vs Input Voltage,Vs Application Note AN213, / 20

10 Figure 6 Dimming Waveforms Figure 7 Maximum Contrast Ratio vs Dimming frequency (100:1=1% duty) Figure 8 Analog Dimming Characteristic Application Note AN213, / 20

11 2.4 Analog Dimming Characteristic The analog dimming characteristic graph is shown Figure 8. To achieve a linear change in LED current versus control voltage, the recommended range of voltage at en/pwm pin is from 1.0V to 2.0V. Table 3 Percentage of max LED current vs DC voltage at PWM pin Ven_pwm /V Percentage of max. LED Current / % < > For application on EN/PWM < 1.0V The threshold of the EN/PWM voltage is around 0.7V. When operate the ILD4001 s EN/PWM pin below the threshold voltage, the output Vdrive become high impedance state and the charges on the gate of external MOSFET are not fully discharged. If the system is powered up at EN/PWM < 1.0V, there are possiblities that the gate charges will turn on the external MOSFET and cause a large current flowing through the LED load. To avoid this, it is advised to add a simple circuitry to prevent the excessive gate charges turn on the external MOSFET as shown in Figure 9. Figure 9 Additional circuitry for EN/PWM < 1.0V Application Note AN213, / 20

12 The extra components R4, R5, T2 and T3 form a simple pull low circuitry when EN/PWM is less than 0.7V. For EN/PWM less than 0.7V, the T2 is off; the T3 is on and pull the gate of T1 to low. This prevents the gate charges turn on the T1 while the Vdrive is in high impedance state. When the EN/PWM is more than 0.7V, the T2 is conduct and pull low the base of T3 which turn the T3 off. And the ILD4001 start working in normal condition. An alternative idea is shown in Figure 10. Figure 10 Alternative circuitry for EN/PWM < 1.0V 2.5 Setting the nominal LED current The internal reference for the voltage across the external sense resistor was design to be 0.116V as stated in the datasheet. A first order approximation for the LED current can be calculated with this formula: I LED V = R sense sense 0.116V = R sense If a certain level of LED current is desired; the estimation for the Rsense is given by: R sense V = I isense LED 0.116V = I LED The Vsense can vary depending on the number of LEDs and voltage supply. Please take reference from Figure 11 and Figure 12 to help select the Vsense for your application. Application Note AN213, / 20

13 3 Characteristic Graphs for different Inductors, no. of LEDs, Rs 3.1 Vsense, ILED versus Supply Voltage Characteristics Figure 11 Vsense vs Vs (Rs = 0.159, L = 68uH) 1 Figure 13 ILED vs Vs (Rs = 0.159, L = 68uH) 1 Figure 12 Vsense vs Vs (Rs = 0.078, L = 33uH) 1 Figure 14 ILED vs Vs (Rs = 0.078, L = 33uH) 1 1 Using BSP318S as external MOSFET Application Note AN213, / 20

14 3.2 Efficiency versus Supply Voltage Characteristic Figure 15 Efficiency vs Vs (Rs = 0.159, L = 68uH) 1 Figure 16 Efficiency vs Vs (Rs = 0.078, L = 33uH) 1 1 Using BSP318S as external MOSFET Application Note AN213, / 20

15 3.3 Temperature Characteristics (Rs=0.157Ω L=68uH) Figure 17 ILED vs Ambient Temperature Figure 19 Solder Point Temperature vs Ambient Temperature. Figure 18 Efficiency vs Ambient Temperature Application Note AN213, / 20

16 4 Evaluation Board and layout Information Figure 20 Photograph of Demo Board using BSP381S external MOSFET (size of PCB: 50mm x 30mm) Figure 21 PCB Layer Information Top View Figure 22 PCB Layer information Bottom View (unflip) Application Note AN213, / 20

17 4.1 PCB Consideration The free-wheeling diode s path from inductor to Vs pin of the integrated circuit is recommended to be as short a distance as possible. This is to minimize oscillation in the system. The energy storage capacitor between Vs and Gnd is recommended to be placed as near to the IC as possible. This helps to stabilize the supply voltage when the IC draws large instantanoeus current during switching. Ground plane should be as large as possible to improve heat dissipation. As a reference for designing the surface area for the grounding for the PCB using FR4 to achieve a certain thermal resistance between desired solder point temperature and expected ambient temperature, the following chart can be used. Figure 23 Thermal Resistance of PCB-FR4 versus Ground Copper Area The data in the above figure 23 were measured with following conditions: Two copper layers. 2 oz copper (70um thick) and board thickness of about 1.6mm. Ground pin connection of the IC is used to dissipate heat. FR4 material. No forced convection. No heat sink. No special mask opening for improved heat dissipation. In the chart, only three points are marked by diamond symbol. These are measured data. The broken line represents intermediate points which can de derived by linear interpolation. Application Note AN213, / 20

18 An example where ILD4001 s PCB is separated from LED PCB and there is not heat transmission between the two PCBs. Figure 24 Thermal Resistance Tj is the junction temperature of the ILD4001 s output transistor connected to switch pin. Ts is the soldered temperature of the ILD4001 s ground pin to FR4-PCB. Ta is the ambient temperature. Rth_js is the thermal resistance from junction to soldered point with reference to ILD4001 s SC-74 package. This is stated as 75K/W in the datasheet. Rth_sa is the thermal resistance from soldered point to ambient which is dependent on size of grounding area of PCB. Pd is the power dissipated by ILD4001 which is approximately 10% of total power from supply (for rough calculation), or it can be derived by (Total power from supply LEDs power Power Loss on other external components). The above variables are related in the equations on the next line. P d T j = R T s th _ js Ts = R T a th _ sa With the above equations, and setting Tj (recommended to be below 100 C), the Ts can be calculated. By choosing a desired Ta, the Rth_sa can be calculated. With the calculated Rth_sa, reference figure 23 to correlate the approximated ground copper area required in PCB layout. 5 Equations for estimating switching frequency or Inductance 5.1 Estimation of switching frequency Table 4 Steps to calculate the switching frequency for the demo boards Board Versions 700mA Units Force Vs = input voltage = 12 V Assume Vrsense = voltage acoss sense resistor = V Assume V LED = avg voltage of one LED = 3 V Assume N = number of LEDs = 3 pcs Assume V LEDxN = voltage across LED+, LED- = 9 V Assume V T = voltage at Vswitch (low state) = 0.1 V Application Note AN213, / 20

19 Table 4 Steps to calculate the switching frequency for the demo boards Board Versions 700mA Units Assume V D = on-voltage of schottky diode = 0.38 V Use Rsense = effective sense resistance = ohm Use L = Inductance = 68 uh 1.) Set LED average current I_average I_ LEDavg = Vrsense / Rsense = A ratio of (Peak to Peak change of LED current) to(average LED Ratio factor = current) = I_ LED = factor * I_ LEDavg = A 2.) Determine SW pin Duty cycle, Dsw V LEDxN + V D + Vrsense Dsw = = V S - V T + V D 3.) Estimated typical operating frequency Dsw (V LEDxN + V D + Vrsense) f = * = factor*l I LEDavg Ratio 195 khz The inductance, L can be make the subject of equation in step 3; given the desired switching frequency, f. Application Note AN213, / 20

20 w w w. i n f i n e o n. c o m Published by Infineon Technologies AG AN213